Extreme heat and the ongoing aridification of the West

Something highly unusual is happening in the western United States. Record hot temperatures, some above 115 degrees Fahrenheit, recently spanned the Southwest northward into the interior West, and now a dangerous heatwave is hitting the Pacific Northwest. The news has been full of references to ongoing western drought and predictions of yet another extreme wildfire year. Even so, in reality, the story is worse.

Much of the West — from California east to New Mexico and north into Canada — is now plagued by extreme and exceptional drought. Precipitation, especially of the frozen winter variety most important to water supplies and forest health, has been well below normal in many locations, and this moisture deficit has been exacerbated by above-normal, increasingly extreme warmth. What’s worse is that the current summer of drought is merely the most recent manifestation of the nation’s first recorded “megadrought,” — a 22-year drought that extends back to the turn of the century.

To most people, droughts are periods of below-normal precipitation that persist until snow and rain returns to normal and the drought is over. To some degree, we have seen such precipitation deficits in the West over the last two decades, especially in the Southwest corner of the country. Not every year has been dry across this region, but the dry years, and especially the bad snow years, have been more common than wet years. Critically, it’s the snow, acting as a natural reservoir, that is most important to the soils, vegetation and rivers of the West.

But the Southwest megadrought, as well as other ongoing droughts across the West are not like the typical droughts of yore. Instead, 21st-century droughts have been “hot droughts” made more severe and lasting by the steadily warming temperatures associated with human-caused climate change. Few alive today lived through “Dust Bowl” drought of the 1930’s, but the worst part of that drought was also the hottest part. And now temperatures are even warmer. Droughts and their impacts are getting even hotter, and thus worse.

Most gardeners know that their plants need extra water on the hottest days, or else the plants will wilt, or even die. This is because a warmer atmosphere will demand more moisture from the soil and plants. And this is exactly what’s happening in the West — as we get warmer temperatures, soils and vegetation lose more moisture to the atmosphere, which because of global warming just keeps getting warmer and more demanding of moisture. In some cases, trees of the West are now dying outright because of unprecedented hot drought. In other cases, trees are being stressed by hot drought, and are then less able to fend off insects and disease that in turn kill off vast swaths of forest trees. And in the most devastating cases, the unprecedented drying of forests by warming has resulted in the unusually severe large and deadly wildfires that are becoming more commonplace across the West.

The warming of the West is doing more than just increasing the odds of forest death and extreme wildfire. Warmer temperatures are also causing river flows to decline, just as they are simultaneously causing the demand for water to increase. Flow in the Colorado River — the most important river of the Southwest —has declined by nearly 20 percent since the turn of the century, and much of the flow reductions were due to warming temperatures rather just than precipitation declines. The same holds true for the Rio Grande of New Mexico and Texas, another river that sustains a critical portion of the drought-plagued Southwest.

As a climate scientist who lived most of his professional life in the Southwest, I was often asked if we were in a megadrought. It wasn’t a relief when I could finally answer “yes.” Now the big question is when will the megadrought end? My answer to this question takes more explanation.

Just as when you go to the doctor with an ailment, the key to knowing what’s ahead is understanding what is causing the problem. In the case of increasing extreme heat and megadrought in the West, we know with high confidence that warming temperatures are the major culprit, and that temperatures will only get hotter until we stop adding heat-trapping gases to the atmosphere — most importantly, from the burning of fossil fuels. We also know that decreasing precipitation is occurring across the Southwest, and it appears this too may continue until we halt human-caused global warming.

Earlier droughts, including megadroughts that predated the arrival of Europeans in North America, resulted mostly from a persistent lack of rain and snow. In the era of human-caused climate change, however, this is no longer true. The relentless warming of the planet and the West is not just worsening the Southwest megadrought (and other regional droughts), it is also pointing to dramatic water shortages and ever-worsening wildfire risks.

We know that the future climate at any spot on the planet will be an interaction of natural climate variability and human-caused change. For the West, this means there will still likely be periods of above — and below — normal precipitation due to natural variations in climate, but it also means that the relentless warming and its disastrous consequences will continue as long as we fail to curb global warming. 

The real story for the western United States is that human-driven climate change, mainly caused by the burning of fossil fuels, will cause ever-worsening heat waves, drought, forest death and water shortages. Yet, there is hope. To stop the aridification of the West, we can wean ourselves from fossil fuels, and halt human-caused global warming.

Jonathan Overpeck is a climate scientist, professor and dean of the School for Environment and Sustainability at the University of Michigan. He has researched drought, climate variability and climate change on five continents. Follow him on Twitter: @GreatLakesPeck.